1. Field of the Invention
The present invention relates to a conditional access system allowing only specific receivers to receive video and audio digital signals which are broadcast or delivered via media such as satellite waves, terrestrial waves or cables.
2. Description of Related Art
FIG. 12 is a block diagram showing a conventional conditional access system described in Telecommunications Technology Counsel Report, Advisory No. 74. In this figure, the reference numeral 1 designates a system controller for generating and managing various keys and subscriber contract information; 2 designates a scrambling processor for encrypting coded media data output from sources like encoders on a transmitting side using scrambling keys output from the system controller 1; 3 designates an ECM (Entitlement Control Message) generator for generating an ECM from the scrambling keys used for encrypting the coded media data and from program associated information including a broadcasting type of a program and charge information: and 4 designates an ECM encryption section for encrypting the ECM using work keys output from the system controller 1.
The reference numeral 5 designates an EMM (Entitlement Management Message) generator for generating an EMM from the work keys used for encrypting the ECM and from subscriber contract information for each receiver; 6 designates an EMM encryption section for encrypting the EMM using a master key output from the system controller 1; 7 designates a multiplexer for generating a packet stream by multiplexing encrypted coded media data, ECM and EMM, and for supplying the packet stream to a transmission path 8 for transmitting the packet stream.
The reference numeral 9 designates a demultiplexer for demultiplexing the packet stream transmitted from the transmitting site into the encrypted coded media data, ECM and EMM; 10 designates a master key managing section for storing in advance a master key used for decrypting the EMM; and 11 designates an EMM analyzer for retrieving from the EMM the subscriber contract information and work keys using the master key stored in the master key managing section 10. The EMM analyzer 11 outputs the work keys only when the subscriber contract information authorizes viewing.
The reference numeral 12 designates an ECM analyzer for decrypting the ECM using the work keys supplied from the EMM analyzer 11, and for retrieving the scrambling keys from the ECM; 13 designates a descrambling processor for descrambling the coded media data using the scrambling keys supplied from the ECM analyzer 12; 14 designates a media decoding section for decoding the coded media data descrambled by the descrambling processor 13; and 15 designates a television receiver for displaying pictures and outputting voices in response to the coded media data decoded by the media decoding section 14.
Next, the operation of the conventional conditional access system will be described.
First, on the transmitting side, the scrambling processor 2 in the transmitting site encrypts the coded media data output from encoders or the like when the coded media data is specific coded media data to be scrambled, such as pay programs.
The ECM generator 3 places into the ECM the scrambling keys used for the encryption, the program associated information indicating the broadcasting type of the program, etc., and the ECM encryption section 4 encrypts the ECM. However, it usually does not encrypt the program associated information in the ECM.
The EMM generator 5 places into the EMM the work keys used in the encryption, the subscriber contract information for each receiver, etc., and the EMM encryption section 6 encrypts the EMM using the master key prepared for the individual receivers. However, it usually does not encrypt the subscriber contract information in the EMM.
Then, the multiplexer 7 multiplexes the encrypted coded media data, encrypted ECM and encrypted EMM, and supplies the multiplexed data to the transmission path 8, through which the various pieces of the information are transmitted as a packet stream.
The packet stream sent from the transmitting side passes through the transmission path 8, and is received by the demultiplexer 9 on the receiving side. On the receiving side, the demultiplexer 9 in the receiving site extracts the encrypted EMM from the packet stream, first. Then, the EMM analyzer 11 decrypts the EMM using the master key stored in the master key managing section 10 for individual receivers, and retrieves from the EMM the subscriber contract information and work keys (since the subscriber contract information is not usually encrypted, it can be referred to without the decryption).
The EMM analyzer 11 supplies the work keys to the ECM analyzer 12 only when the subscriber contract information authorizes the viewing.
Receiving the work keys from the EMM analyzer 11, the ECM analyzer 12 decrypts the encrypted ECM using the work keys, retrieves the scrambling keys from the ECM, and supplies the scrambling keys to the descrambling processor 13.
Receiving the scrambling keys from the ECM analyzer 12, the descrambling processor 13 descrambles the coded media data using the scrambling keys, and the media decoding section 14 decodes the coded media data descrambled by the descrambling processor 13.
The decoded coded media data is transferred to the television receiver 15 that displays pictures and outputs voices.
With the foregoing configuration, the conventional conditional access system authorizes only specific users' receivers to descramble the coded media data in accordance with the contents of the subscriber contract information. The receivers of unauthorized users, however, cannot retrieve the scrambling keys, and hence the unauthorized users cannot view programs at all. Thus, they cannot view even fragments of pictures of programs, and hence cannot have slightest feelings about the programs as in analog broadcasts. As a result, they cannot decide as to whether they make a new contract or not after trial viewing of some programs, which prevents the providers from making effective invitation to their programs.